The bronze birch borer is a native insect which occurs throughout the range of birch in North America. It is a serious pest of ornamental birch plantings.

Description:

The bronze birch borer is a slender, olive to copper-bronze colored beetle, 6-12 mm (1/4-1/2 inch) long. Females are slightly larger than males. The larvae are white, slender, and legless with a light-brown head capsule that is retracted somewhat into a wide, flattened first-thoracic segment. The remaining body segments are smaller and ribbon-like in appearance. Two brown, hardened, pincer-like structures are located at the tip of the abdomen. Mature larvae may be up to 12 mm (1/2 inch) long.

Damage:

The bronze birch borer attacks paper (canoe), European white (especially cutleaf variety), gray, yellow, and other birches, as well as poplar, cottonwood and willow. The larva bores in the trunk and larger limbs, often girdling them. Trees weakened by drought or injured are most susceptible to attack.Tree injury is caused by larval tunneling in the inner bark or cambium. The girdling of the trunk or branches interrupts sap flow downward to the roots and destroys the tree's cambium tissue. The interruption and subsequent accumulation of sap flow above larval tunnels often causes characteristic swollen bands or vein-like ridges in trunks and affected branches. The galleries range to approximately 1.5 meters (5 feet) in length and are packed with a dark brown sawdust-like frass.

The first sign of bronze birch borer attack is usually a die-back of the uppermost branches, followed by a gradual decline and eventual death of the entire tree in two to three years. The presence of "D" shaped adult-emergence holes in the tree trunk is a positive sign of borer activity. Attack usually begins in crown branches 19 to 25 mm (../images/4 to 1 inch) in diameter, progressing downward year after year. In healthy, vigorous trees, the larvae do not complete development and the galleries heal over.

Life Cycle:

Adults begin to emerge through "D" shaped holes in the bark in early to mid-June and may continue for five to six weeks. They feed on the foliage of alder, aspen, birch, willow or poplar for at least a week before beginning to lay eggs in bark crevices or beneath loose bark. Like other flatheaded borers, the bronze birch borer is attracted to the sunny side of trees for feeding and egg laying (oviposition). Areas of recent injury, mechanical or other, appear most attractive to the borer. Females lay up to 76 eggs, which hatch in two weeks. The larvae bore into the inner bark (phloem) and cambium to feed. Mature larvae construct oblong cells in the thick bark or wood in the fall and pupate the following spring. There is only one generation per year.

Control:

Cultural:

Avoid planting susceptible (white-barked) birch species. European white birch and its cutleaf cultivar are most susceptible to attack. River birch, Betula nigra, has salmon-pink bark and is resistant to the bronze birch borer. 'Heritage' is a relatively white-barked cultivar of river birch which is readily available in clump form and as single stems. The U.S. National Arboretum and others have breeding programs to develop resistant birches with desirable landscape characteristics.

Improper site selection for planting birch trees often contributes to the borer attack and death of the trees. Under natural conditions, birch grows best in cool, moist, shaded situations. It is not adapted to open, sunny, exposed locations, such as the middle of a large, open yard or the exposed south or west side of a building. Trees planted in such sites will lose vigor and become weakened, increasing their susceptibility to borer establishment. Adult borers also prefer to lay eggs on trees in full sunlight.

It is possible to prolong the life of susceptible trees by reducing stress. Deep watering may be helpful during dry periods. Trunk injury by lawn mowers, etc. can also weaken the tree and provides an attractive bronze birch borer oviposition site. Mulching around the tree base eliminates the need for close mowing and potential trunk injury and retains soil moisture.

Bronze birch borer-infested firewood should be burned before May to stop adults from emerging.

Chemical:

Chemical control is difficult once larvae have bored into the trunk. Use foliar and bark insecticide sprays to control egg-laying adults or newly-hatched larvae before they enter the bark. Apply the first spray thoroughly to the bark and foliage in early June when Kousa dogwood blooms. Since the adult flight period may last for six weeks or more, two additional sprays at 3-week intervals are suggested. Most effective insecticides for the bronze birch borer are restricted to professional applicators. Homeowners may find permethrin available at some garden centers.

Adapted from the University of Vermont Cooperative Extension, 1999